1. Field of the Invention
The invention relates generally to the field of digital radio signal communications using adaptive antenna arrays. More particularly, the invention relates to deriving a transformation function to generate transmit waveform for a system user terminal using a waveform received from the system user terminal.
2. Description of the Related Art
Radio communications capacity can be greatly increased using directional, rather than omni-directional radio transmission. One way to transmit directional signals and directionally receive signals is by using beam forming and nulling through an array of antennas. Other antenna array strategies can be combined with beam forming and nulling when appropriate to the application.
The precision of the beam forming and nulling using an antenna array, can be improved if the transmit and receive chains, from the digital interface at baseband to the field radiated from each antenna element is accurately calibrated. Errors can arise from differences between the antenna elements as well as differences in the hardware used for phasing, amplifying and modulating or demodulating communicated radio signals, among other things. One way of making the calibration is to have a remote, portable RF transponder listen to the output of the antenna array on a base station downlink frequency and re-transmit the downlink signal that it receives from the base station, on the base station""s uplink frequency. By selecting appropriate signals to transmit and appropriate signals to receive, the base station can apply signal processing to estimate compensations in phase and amplitude to calibrate its transmit and receive chains. Such a technique requires external equipment including additional radios and either directional couplers or additional antennas mounted somewhere visible to the base station.
The precision of the beam forming and nulling from an antenna array can also be improved if the parameters of the radio channel to the subscriber can be accurately characterized. Since the subscriber may be moving and the signal propagation channel may be changing, precision is improved with frequent channel estimates. The channel can be characterized using the response of the antenna elements to a signal received from the subscriber, for example, by estimating the phase and amplitude response of the receive section of each antenna element. The phase and amplitude responses can be expressed as an uplink spatial signature for a particular user and a particular array. The uplink spatial signature can be used to generate a downlink spatial signature, which can be used in directing a beam to the subscriber over the same signal propagation channel used for reception. Accurately converting an uplink signature to a downlink signature enhances the accuracy of many directional techniques that can be performed with an antenna array.
Constantly and consistently making measurements that provide a high level of precision for both the calibration and the uplink to downlink transition can require a significant amount of processing power. It may also require additional equipment. If the precision of these measurements is not maintained, then service quality and system capacity can be reduced.
A method and apparatus are provided that allows system calibration to be incorporated into a single function that also derives transmit parameters from;receive parameters in a spatial diversity system. In one embodiment, the invention includes receiving signals at an antenna array from a plurality of different locations, deriving characterizations of the spatial parameters of the received signals, receiving measurements of a plurality of different signals transmitted from the antenna array to a plurality of different locations, deriving characterizations of the spatial parameters of the transmitted signals from the received measurements, and generating a transformation function for producing transmit spatial parameters based on measurements of received signals using the receive spatial parameter characterizations and the transmit spatial parameter characterizations.
Other features of the present invention will be apparent from the accompanying drawings and from the detailed description that follows.